new Infusoria from American Fresh Waters. 439 



extremities rounded, the anterior the narrower ; ventral sur- 

 face concave, the dorsal elevated into a single, median, longi- 

 tudinal, keel-like projection, traversing the entire body ; 

 flagellum not exceeding the body in length, arising from the 

 ventral surface somewhat back of the frontal border ; oral 

 fossa capacious ; nucleus near the centre of the right-hand 

 margin, the contractile vesicle somewhat in advance, on the 

 opposite side ; parenchyma transparent. Length of body 

 ttW inch. 



Hah. Standing pond-water, with aquatic plants. 



This form seems to combine the characters of P. abscissa 

 (Duj.), Stein, and of P. mediocanellata, Stein, the former 

 bearing one or two dorsal keel-like elevations, and the latter 

 having a groove traversing its ventral surface, while P. cari- 

 nata possesses both in a marked degree. The dorsal aspect 

 of the latter is conspicuously angular, the keel-like ridge 

 forming the apex and the right-hand and left-hand sides 

 respectively sloping evenly in opposite directions to the lateral 

 borders, as shown in diagrammatic transverse optic section by 

 fig. 14. The infusorian is much the smallest member of the 

 genus hitherto observed. Its movements are usually directly 

 forward and not rapid, the flagellum conspicuously vibrating 

 only at its distal extremity, the creature frequently coming to 

 rest on a fragment of aquatic plant, and extending the flagel- 

 lum in all directions, as if in search of food, or, where food 

 seems specially abundant, remaining for a long time with the 

 anterior border or the large oral aperture in contact with the 

 heap of ddbris, the posterior extremity being lifted upwards, 

 the flagellum then also being directed to various points in 

 the vicinity. 



Zygoselmis acus, sp. nov. (PI. XV. fig. 15.) 



Body elongate, needle-shaped or subfusiform, changeable in 

 shape, about six times as long as broad, both extremities 

 pointed ; surface smooth ; endoplasm granular ; flagella very 

 diverse in length, the shorter scarcely as long as the body, 

 the longer once and a half to twice that length ; nucleus 

 apparently subcentral. Length of body T5V0 inch. 



Hab. Standing pond-water, with aquatic plants. Move- 

 ments active. 



There seems to be but little dissimilarity between the 

 Zygoselmis of Dujardin and Ehrenberg's Distigma, the 

 possession by the latter of two eye-like pigment-spots beino- 

 the chief point of difference, and even these often being absent. 

 In Zygoselmis they have not been observed at any stage of 



